Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/symfony/debug/Tests/HeaderMock.php

@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Debug;
+
+function headers_sent()
+{
+    return false;
+}
+
+function header($str, $replace = true, $status = null)
+{
+    Tests\testHeader($str, $replace, $status);
+}
+
+namespace Symfony\Component\Debug\Tests;
+
+function testHeader()
+{
+    static $headers = array();
+
+    if (!$h = func_get_args()) {
+        $h = $headers;
+        $headers = array();
+
+        return $h;
+    }
+
+    $headers[] = func_get_args();
+}